Which of these numbers is prime? ${55,\ 65,\ 71,\ 81,\ 86}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 55 are 1, 5, 11, and 55. The factors of 65 are 1, 5, 13, and 65. The factors of 71 are 1 and 71. The factors of 81 are 1, 3, 9, 27, and 81. The factors of 86 are 1, 2, 43, and 86. Thus, 71 is a prime number.